[seqfan] Number of indecomposable permutations sortable using two parallel stacks

[Charles Greathouse]

Don Knuth points out that
https://oeis.org/A215252
and
https://oeis.org/A215257
have the same name, and suggests explaining them well enough to distinguish
the two. Alois P. Heinz suggests that the latter uses a double-ended queue:
https://oeis.org/draft/A215257

Would anyone familiar with these sequences comment or verify this?

Charles Greathouse
Analyst/Programmer
Case Western Reserve University